During the 1850s, Pleasants had worked for the Pennsylvania Railroad and participated in digging a lengthy tunnel through the Allegheny Mountains. Before the outbreak of war, he had worked as a mining engineer in Schuylkill County.

In June 1864, Pleasants suggested to his commanding officers that the strongly entrenched Confederate lines near Petersburg could be breached by digging a mine and placing explosives directly underneath the Confederate position. Gens. Burnside, George G. Meade and Ulysses S. Grant all approved the plan.

Pleasants’ men began digging the mine on June 25, 1864. Pleasants, who was afforded little help by skeptical Army engineers, devised successful methods for ventilating the shaft and disposing of the dirt during the digging.

In less than a month, the Union “soldier-miners” completed a 510-foot tunnel to the Confederate position on a ridge known as Elliot’s or Pegram’s Salient. At the end of the tunnel, the troops dug a 75-foot shaft underneath the salient, where they placed 4 tons of explosive powder.

The explosive powder was ignited shortly before 5 a.m. on July 30. One Union soldier recalled the scene:

“Suddenly the earth trembled under our feet. An enormous mass sprang into the air. A mass without form or shape, full of red flames, and carried on a bed of lightning flashes, mounted towards heaven with a detonation of thunder.

“It spread out like a sheaf, like an immense mushroom whose stem seemed to be of fire and its head of smoke. Then everything appeared to break up and fall back in a rain of earth mixed with rocks, with beams, timbers, and mangled human bodies, leaving floating in the air a cloud of white smoke, which rose up in the heavens, and a cloud of gray dust, which fell slowly towards the earth.”

Another Union soldier described it as a “tremendous blast which rent the sleeping hills beyond, a vast column of earth and smoke shoots upward to a great height, its dark sides flashing out sparks of fire, hangs poised for a moment in mid-air, and then, hurtling down with a roaring sound, showers of stones, broken timbers, and blackened human limbs.”

Historian John M. Taylor surmises that “it may have been the greatest man-made explosion up to that time.”

The blast created a crater that measured about 170 feet long, 70 feet wide and 30 feet deep in places. The Confederates lost 278 men as a direct result of the explosion. Many other Confederate troops fled the area immediately after the blast.

Thus a huge gap, ripe for exploitation, had been created in the Confederate network of trenches near Petersburg. If Petersburg fell, Richmond, the Confederate capital, could not hold out for long. In the words of historian Bruce Catton, “The Pennsylvania miners had brought the end of the war within whispering distance.”

Burnside’s plan originally called for a division of black troops under the command of Gen. Edward Ferrero to rush through the gap created by the explosion and take the heights overlooking Petersburg. Meade, concerned about possible criticism should the black troops fail and be slaughtered in their first combat action, at the last minute ordered Burnside to select another division to make the initial charge. Grant agreed.

Burnside unwisely selected a division led by Gen. James Ledlie, described by Mr. Catton as the “weakest” and “most unfit” of Burnside’s division commanders. The result, Grant recalled in his memoirs, was “a stupendous failure.”

Ledlie’s troops, with their commander drinking rum a safe 400 yards behind the line, rushed toward the crater and, instead of moving around its edges toward Cemetery Hill to exploit the breach in the line, entered the crater in mass confusion.

Confederate forces regrouped and poured artillery fire into the crater with deadly effect. Confederate Gen. William Mahone organized a counterassault that occupied the rim of the crater, where his troops fired into the mass of Union soldiers.

“Men who got out alive,” Mr. Catton wrote, “remembered a horrible debris of severed limbs and heads flying through the air after each shell exploded.”

The gap in the Confederate lines had closed. By 1 p.m., the battle was over. The siege of Petersburg would continue for another eight months.

Today, 141 years later, the crater is still there, partially outlined by a wooden railing at the crest of a gradually sloping hill at Petersburg National Battlefield Park. Historian Shelby Foote aptly described the crater as a “raw scar” that greened over and lost its jagged look, but never really healed.

At the base of the hill, you still can see the entrance to the tunnel dug by Pleasants and his Schuylkill County miners. Pleasants’ ingenious plan, which presented an opportunity to end the war in 1864, instead resulted in a Union disaster because of poor execution and leadership. The Battle of the Crater is a useful reminder that few plans survive unscathed by the fog of war.

 Francis P. Sempa is the author of “Geopolitics: From the Cold War to the 21st Century.”